Because my game group likes 3e and its children (I'm going to refer to them all as 3e because that is easier).
That's basically the only reason. Other than that I would rather play any version of D&D - possibly excepting 4e, which I have never played but does not seem particularly appealing by all accounts. In fact I don't even own any 3e books, all my Pathfinder books are borrowed from one of my players. I own the rules of course to four other editions, and I find lots of uses for them nonetheless.
I don't like 3e. From the first time I saw it when it came out, to actually playing in a few campaigns (of 3.5) to running now my third Pathfinder game I think it is less fun than old-school D&D.
Skills - for all the reasons brilliantly explained by -C here, I think skills basically reduce fun.
Example from my last game session: unknown monster appears in the dungeon (fungal crawler), player rolls knowledge dungeoneering and rolls well. I am faced with a choice: deny them information to preserve the mystery of the dungeon, and punish them for taking "soft skills", or give them information. Well I approve of developing characters with aims other than maximum damage output, so I tell them some tidbits about the fungal crawler, I try to couch it in terms of "you can tell by its grasshopper legs that it is probably quite a jumper" but wouldn't it be more fun to find out first hand?
In theory skills seem great, but in my last days of playing 2e (the rule set I cut my teeth on and played the most of) I dropped nonweapon proficiencies (yes, they are an optional rule!) in favor of secondary skills (which I think are dandy).
Where I like skills is Stars Without Number, but that is a different game and a topic for a different day.
Feats - Unlike almost everyone I know, I hated feats from the moment I saw them. Feats are the reason I never adopted 3e in its heyday.
My feelings towards them have soften somewhat since, but I looked at them at the time and thought: wow, this is terrible. What do feats give us theoretically? Control, character customization, options. But the raise a couple of big problems: first of all, people no longer feel that they need to distinguish their character through roleplaying, now they are distinguished largely by their class powers and feats. ("What are you playing?" [class and list of feats]).
Second, they heavily reward power gamers. In pre-3e D&D power gaming was only marginally rewarded, now player skill at character building leads to great discrepancies in power. The characters of players played by more experienced (or more power game focused) players are mechanically more powerful than those of other players. This is very different than a difference in player skill as thought of in the OSR. In old school D&D you don't know what you are doing, so you kill your character, roll up a new one and learn. In new school D&D you don't know what you are doing so your character sucks and does not get to participate at the same level as other player's characters. This of course can only be remedied by making a new more powerful character. So if you want to have the same amount of fun as the rest of the group, you need to make sure you are carefully optimized as well. One could easily say this is a metagame problem with particular groups, but the root problem is that 3e rewards powergaming so heavily. You tend to get what the game rewards.
More to come.